1. Field of the Invention
The present invention relates to an image data encoding/decoding method and apparatus, and more particularly to an image data encoding/decoding method and apparatus which are capable of efficiently performing compression for natural gray scale images and binary images.
2. Discussion of the Background
Various image processing apparatuses, such as printers, digital copying machines, facsimile machines, scanners, image filing systems, digital cameras, digital video recorders, CD-ROM players, and so forth, have been using image data encoding/decoding apparatuses. One type of the image data encoding/decoding apparatuses is based on a frequency data conversion method by which data are converted according to frequency magnitude. Several techniques are known as the frequency data conversion method. For example, a DCT (discrete cosine transform) and a Harr Wavelet conversion method, performing image data conversion by dividing data into a plurality of data blocks according to magnitude of frequency in horizontal and vertical directions, are significant as being capable of compressing an image of a natural gray scale in an efficient manner. As another example, Japanese Patent Application Laid Open No. 2-222386 discloses a technique which extracts edge blocks from image data before performing the DCT and provides a relatively large amount of data to the edge blocks.
However, as recognized by the present inventor, the above-mentioned image data compression methods have a drawback in that compression efficiency for a simple binary image, such as a computer graphic, text, and so forth, is relatively low. Accordingly, a printer, for example, implementing one of the above-mentioned image data compression methods, and which is used in an environment where images of computer graphics and text are mostly required, does not produce images in a superior compression efficiency.
Therefore, at present, there is no image data encoding/decoding apparatus capable of efficiently performing compression for both a natural gray scale and computer graphics and text image(s).